The South Carolina Republican previously said that cutting the national debt would be a major priority, and he suggested during his confirmation hearing that cuts in entitlement programs like Medicare should be on the table.

Mulvaney's history of advocacy for stringent budget cuts generated opposition from Democrats and also Arizona Republican Sen. John McCain, who objected to Mulvaney supporting taking troops out of Afghanistan in 2011 and cutting $3.5 billion from a 2013 defense appropriations bill.

"It's not about one Cabinet position. This is not personal. This is not political. This is about principle,” McCain said on the Senate floor Thursday. “This is about my conviction as chairman of the Senate Armed Services Committee that providing for the common defense is our highest constitutional duty, and that rebuilding our military must be the Number 1 priority of the Congress and the White House."

When he was in Congress, Mulvaney was a member of the House’s conservative Freedom Caucus. The group is critical of government spending and has supported the threat of government shutdowns in budget fights. McCain has been critical of shutdowns as a bargaining chip and said that Mulvaney contributed to Washington’s “dysfunction.”

As director of the White House Office of Management and Budget, Mulvaney will oversee creation of Trump's federal budget plan and also will be the point person for White House efforts to improve government management and reduce waste.

Sen. Mike Enzi, R-Wyo., chairman of the Budget Committee, was critical of how long it had taken to get Mulvaney confirmed and expressed confidence in him as OMB director.

“OMB can play an important role in making sure that spending programs don’t duplicate each other,” Enzi said on the Senate floor Thursday. “I have full faith in Rep. Mulvaney.”

Illinois Democratic Sen. Dick Durbin voted against Mulvaney.

“His priorities as a member of the Freedom Caucus, a founding member, disqualify him for this job,” Durbin said Thursday. “That is the group that led to the resignation of Speaker Boehner and continues to tie the House of Representatives into knots.”